The use of gamma ray scattering measurements to measure the density of subsurface formations is well known. A general introduction to this topic can be found in Well Logging for Earth Scientists, by Darwin V. Ellis, Elsevier Science Publishing Co., Inc., 1987, pgs. 201-212, incorporated herein by reference. Gamma ray density measurement logging tools were initially developed for open hole applications and the earliest single detector tools were soon replaced by dual detector tools that allow compensations to be made for the possible intervening presence of mudcake or drilling fluid between the tool and the formation.
While originally developed for open hole logging purposes, it has long been known that these types of gamma ray density tools could also be used to obtain density measurements through casing. See, for instance, U.S. Pat. No. 4,297,575, issued Oct. 27, 1981; “Gas Detection from Formation Density and Compensated Neutron Log in Cased Hole”, M. Cigni and M. Magrassi, SPWLA 28th Annual Logging Symposium, 1987; “Reevaluation of Hydrocarbon Reserves Through Case-Hole Interpretation: A New Approach”, L. Cosentino and G. Spotti, SPE 22345, 1992; and “Monitoring Contact Movement during Depressurization of the Brent Field”, E. Quint, SPE 56951, 1999, each of which is incorporated herein by reference. While conventional open hole gamma ray density tools have occasionally been used to make formation density-related measurements in cased wells, this technique has not become widely used because the relatively weak gamma ray sources (typically a Cesium-137 source emitting gamma rays at 662 keV) and relatively short source/receiver spacings used in conventional open hole gamma ray density tools results in a relatively shallow depth of investigation, often insufficiently deep to make accurate formation density measurements through casing. In addition, the use of conventional open hole gamma ray density measurement data processing methodologies have not sufficiently accounted for the many types of differing conditions encountered while making measurements through casing, such as variations in apparent casing thickness due to corrosion, casing collars, centralizers, etc.; variations in the thickness of the cement between the casing and the formation; the presence of water, drilling mud, or gas filled voids in the cement layer; etc.
There has recently been work done on the development of a gamma ray density tool specifically designed by operate in a cased-hole environment. See for instance, “Computer Simulation of Cased-Hole Density Logging”, L. Jacobsen and C. Fu, SPE 19613, December 1990; and “Design of a Cased-Hole-Density Logging Tool Using Laboratory Measurements”, G. Moake, SPE 49226, 1998, both incorporated herein by reference. These proposals have primarily focused on the use of a more energetic gamma ray source (such as Cobalt-60) and larger source/receiver spacings to increase the depth of investigation. Although the need for a gamma ray density behind casing measurement is not seriously in doubt, the tool proposed in these papers (or any other tool specifically designed to make gamma ray density measurements behind casing) has never been commercially successful.
Schlumberger, assignee of the present invention, has long been a leader in the measurement of formation density using gamma ray scattering behavior. Early gamma ray density measurement tools are described in commonly-assigned U.S. Pat. No. 3,864,569 to J. Tittman, issued Feb. 4, 1975 and U.S. Pat. No. 4,048,495 to D. Ellis, issued Sep. 13, 1977. Commonly-assigned U.S. Pat. No. 5,390,115 to C. Case and D. Ellis, issued Feb. 14, 1995 and U.S. Pat. No. 5,841,135, to C. Stoller et al., issued Nov. 24, 1998, describe three detector gamma ray density logging tools designed for open hole use that have enhanced abilities to correct for standoff between the tool and the formation. Each of these earlier, commonly assigned patents are incorporated herein by reference.
Schlumberger has successfully introduced a three detector gamma ray density logging tool called the TLD* (* Mark of Schlumberger) as part of the Platform Express* triple combo tool. The TLD* tool has a Cesium-137 source and three detectors, a backscattering detector located less than two inches (5 centimeters) from the source, a short spacing detector located approximately 6-8 inches (15-20 centimeters) from the source, and a long spacing detector located approximately 14-18 inches (36-46 centimeters) from the source. The diameter of the TLD* tool when closed is approximately 4¾ inches (12 centimeters).